"Depositional, Mineralogical, and Maturity Controls on Pore Types, Porosity, and Pore-Size Distribution in Mudrocks"
Lucy Tingwei Ko
Jackson School of Geosciences The University of Texas at Austin
Mudrocks are seals and barriers for conventional petroleum systems and are sources, seals, and reservoirs for unconventional petroleum resources. They are also candidates for nuclear-waste disposal and carbon capture and storage sites. This investigation improves understanding of the interplay of depositional, diagenetic, and hydrocarbon generation processes in the origin and evolution of OM-rich mudrock pores by providing (1) a process-based understanding of the origin and control of different pore types, (2) quantitative methods to characterize pore systems in mudrock reservoirs, (3) a link between sediments and depositional processes to pore system variations, and (4) a comparison of pore evolution history in marine and lacustrine mudrocks.
Laboratory study of gold-tube pyrolysis using immature Barnett, Eagle Ford, and Woodford mudrock samples to induce and observe changes of organic matter and pores at different levels of thermal maturity led to the development of pore evolution models of mature mudrocks and a classification scheme of OM pores. Four main categories of OM pores comprise the OM-pore network: primary OM pore, convoluted OM pores, OM bubble pore, and OM spongy pore, with the latter two related to thermal maturation processes. The original interparticle and intraparticle mineral pores, which are a function of depositional and early diagenetic processes, determined the mineral pore network before petroleum generation and migration began. The mineral pore network preset and constrained petroleum migration and OM redistribution, controlling the modified-mineral-pore and OM-pore network after thermal maturation of OM. When OM became mature, the pore evolution was closely related to OM conversion. The morphology of mineral and OM pores varied with the stages of OM maturation. Predominant pore types changed from primary mineral pores, to modified mineral pores with relic OM, to coexisting modified mineral pores, OM bubble pores, and OM spongy pores, and finally to OM spongy pores.
By studying several suites of subsurface marine (Eagle Ford, Pearsall) and lacustrine (Yanchang) core samples, multiscale heterogeneity of mudrock properties can be defined and parameters that affect the heterogeneity of pores can be determined. Pore sizes in OM-rich mudrocks range across at least five orders of magnitude (from 1 nm to approximately 100 μm). In general, OM pores are smaller than mineral pores. Mudrocks with well-sorted and coarse grains contain more abundant mineral pores than those with poorly-sorted and finer grains. Thermal maturation processes, kerogen and maceral types controlling OM pore development and evolution is recognized by integrating SEM petrography, organic petrography, and geochemistry. Primary mineralogy and texture determine diagenesis such as compaction and cementation and could indirectly affect sizes of OM pores up to two orders of magnitude differences.
About the author
Lucy Ko has been working in the mudrock system research laboratory (MSRL) group as a Ph.D. student here at the Bureau of Economic Geology. She has a multidisciplinary background in geochemistry, geology, basin modeling, and fluid property prediction. She received her B.A. degree from National Taiwan University and her M.S. degree from the Colorado School of Mines, where she utilized petroleum geology, biomarker, gas compositions and isotopes to understand the charging and filling history of the tight-gas reservoirs in the Rocky Mountain Basins. She worked in Platte River Associates (a basin modeling company) for a year and has done two internships at ConocoPhillips in Houston. She is co-supervised by Bob Loucks and Steve Rupple at the BEG but she also works with Harry Rowe, Kitty Milliken, Tongwei Zhang, Sheng Peng, and Farzam Javadpour in the MSRL group. Her PhD research focuses on the depositional, diagenetic, mineralogical, and maturity controls on mineral- and organic-pore networks in marine and lacustrine mudrocks.